As a means for examining and treating the inside of a body cavity or the like by inserting an insert part into the body cavity or the like, a video endoscope is widely used, which has a solid-state image pickup device (hereinafter referred to as “CCD”) incorporated in a distal end portion thereof.
There are two image pickup systems available to video endoscopes using a single CCD.
One of them is a field sequential image pickup system in which R, G and B light are sequentially applied to a subject, and reflected light from the subject is imaged by using a monochromatic CCD.
The other is a simultaneous image pickup system in which imaging is carried out by using a color CCD employing a primary or complementary color filter.
Regarding a subject having white and black rectangular-wave patterns in the horizontal direction, the number of pixels required to make a distinction between a pair of white and black patterns is considered in general as follows.
When a monochromatic CCD is used, the minimum number of pixels required is two.
When a color CCD employing an interline type complementary color filter, which is employed in many video endoscopes, is used, the minimum number of pixels required is three.
For the reasons stated above, 1 pixel in the monochromatic CCD and 1.5 pixels in the color CCD may be regarded as the smallest unit necessary for producing one smallest image in the horizontal direction (hereinafter referred to as “smallest unit pixel” for producing one image in the horizontal direction of the CCD).
Recently, with the reduction in pixel pitch of CCDs, the pixel density has become higher, and endoscopes of high image quality have been realized which are capable of observing even very small portions that have heretofore been impossible to see with conventional CCDs having a pixel pitch of 8 μm or more. On the other hand, diagnosis may be hindered by a scratch or chip on an image pickup optical element or foreign matter attached thereto that are so small as not to interfere with diagnosis in the past because of the high-resolution image quality.
Further, if the focal length decreases or the F-number increases in an endoscope image pickup optical system without an increase in size of the optical system, the light beam incident on the smallest unit pixel for producing one image in the horizontal direction of the CCD becomes smaller in diameter than in the past. Consequently, diagnosis may be hindered by a scratch or chip or foreign matter on an objective optical element that have heretofore presented no problem.
Among the causes of disturbing image quality arising from the presence of the above-described scratch or chip or foreign matter on the image pickup optical element, three causes due to the optical element closest to the object side in the image pickup optical system are frequently regarded as problematic.
Cause 1 is the degradation of image quality due to fogging of the image-side surface of the optical element closest to the object side in the image pickup optical system. When an endoscope is inserted into a body cavity, the contents of the body cavity or the mucus may be attached to the surface of the optical element closest to the object side in the endoscope image pickup optical system (the optical element surface will hereinafter be referred to as “viewing window”), disturbing the distinct vision. The foreign substances attached to the viewing window are washed off with washing water sent from a nozzle for washing provided on the distal end of the endoscope. However, because the washing water is lower in temperature than the viewing window, which has been heated by the body temperature, condensation occurs on the image-side surface of the optical element closest to the object side in the image pickup optical system. Fogging of the optical element makes it impossible to obtain a favorable image. Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 5-281492 mentions a means for preventing fogging, as shown in FIG. 12, by air-tightly sealing the joint 3 between an optical element 1 closest to the object side in an image pickup optical system and an optical element 2 next closest to the object side to prevent water vapor from entering an air layer between the optical element 1, which is the closest to the object side, and the optical element 2, which is the next closest to the object side, thereby preventing occurrence of fogging. However, if the front and second optical elements are cemented together to attain an airtight seal, the adhesive may enter the inside of a bundle of rays (hereinafter referred to as “within the visual field”) incident on the CCD by capillary action, causing flare, which gives rise to a problem. Hitherto, when such problems occur, the image pickup optical system and the image pickup unit are repaired by taking them out. Therefore, it costs a great deal to make repairs.
Cause 2 is the degradation of image quality due to the adhesion of water to the object-side surface of the viewing window. An endoscope is, as shown in FIG. 13, equipped with an objective optical system 6, an illuminating optical system 7, and a nozzle 5. Filth attached to the object-side surface of the viewing window is washed off with washing water from the nozzle 5, and the washing water is blown off by air sent from the nozzle 5. However, the adhesive applied to the periphery of the optical element closest to the object side in the image pickup optical system interferes with the blowing off of washing water and foreign matter by air. Consequently, all the washing water and so forth cannot flow away, but drops of water remain on the periphery of the viewing window and disturb the distinct vision (this will hereinafter be referred to as “poor drainability”). To improve the poor drainability, the viewing window may be covered with a water-repellent coating as in Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 2-129613. However, endoscopes for medical treatment have to undergo severe use environments such as disinfection and sterilization. Therefore, such a water-repellent coating is very likely to separate from the lens and hence cannot be used continuously. Further, in these days of CCDs with a decreasing pixel pitch, the light beam incident on the smallest unit pixel for producing one image in the horizontal direction of a CCD is smaller in diameter than in the past. Accordingly, foreign matter attached to the object-side surface of the viewing window is conspicuous. Under these circumstances, because the light beam becomes thinner at the peripheral portion of the viewing window, which exhibits a strong tendency to disturb the distinct vision owing to the poor drainability, diagnosis may also be hindered by small drops of water remaining on the periphery of the object-side surface of the viewing window, which have heretofore presented no problem.
Cause 3 is the degradation of image quality due to a scratch or chip on the object-side surface of the viewing window. When an endoscope is carried in a hospital or the like, the viewing window may be scratched or chipped if the endoscope is bumped against something by mistake. A small scratch or chip that gives rise to no problem in diagnosis when the light beam incident on the smallest unit pixel for producing one image in the horizontal direction of the CCD corresponds to the order of a pixel pitch of 8 μm or more as in the past is conspicuous and hinders diagnosis because the image is of high resolution.
Such a small scratch or chip is more conspicuous for the reasons stated above when the focal length is 2.2 mm or less or the F-number is 3.5 or more in a small optical system such as an endoscope image pickup optical system.